Common Terrarium Care Mistakes You Might Be Making

You spent time on it. Picked the plants carefully, layered the soil just right, and for a few days, it looked absolutely perfect sitting on your desk or windowsill.

Then something went wrong.

Maybe the leaves started yellowing. Maybe a strange white fuzz crept along the soil. Maybe it just looks sad, and you can’t figure out why. Honestly, terrarium care catches a lot of people off guard, even those who are pretty comfortable with plants or a self-professed green thumb. At J2 Terrarium, after years of running workshops all across Singapore, we’ve seen the same slip-ups come up time and time again, from total beginners to people who’ve been keeping plants for as long as they can remember.

Here’s the thing, though: every single one of these mistakes is fixable. Most of them are easy to avoid once you know what to look out for.

You’re Probably Watering It Too Much

This is the big one. Overwatering is the most common reason terrariums fail, and the frustrating part is that it rarely looks like a problem until it already is.

A closed terrarium isn’t like a regular potted plant. It creates its own little water cycle where moisture evaporates, condenses on the glass, and drips back into the soil by itself. So if you’ve been watering on a regular schedule out of habit, there’s a real chance you’ve been adding water the terrarium never actually needed.

Too much water leads to root rot. Root rot creates the perfect conditions for mould. And in an enclosed space, mould spreads faster than you’d like. For closed terrariums, a light watering every two to four weeks is usually more than enough. For open ones, press a finger into the top inch of soil. If it still feels damp, just leave it a few more days. A misting bottle is worth having because it gives you so much more control than pouring water straight in.

The Wrong Plants Are Sharing the Same Space

Not every plant is suited to life inside a terrarium, and not every terrarium plant belongs in the same one together. This is where a lot of people get caught out.

Closed terrariums suit humidity-loving species like mosses, ferns and tropical plants that thrive in warm, moist conditions. Open terrariums are a much better home for succulents and cacti, which need drier air and room to breathe. Mixing the two might look great on day one, but one group will always struggle for it.

This comes up a lot during our workshops. Someone has a plant they really love and want to include, without realising it has completely different needs from everything else in the build. A few minutes spent checking that your chosen plants share similar light and humidity preferences before you start building saves a lot of heartache later.

It’s Sitting in Too Much Direct Sun

It makes total sense on paper. Plants love sunlight, so a bright sunny windowsill must be ideal, right? The problem is that glass behaves very differently from open air.

Glass traps and intensifies heat. A terrarium left in direct afternoon sun can warm up much faster than you’d expect, scorching leaves and drying out moisture before the plants have a chance to absorb it. The stress this causes isn’t always visible straight away, which is what makes it easy to miss. A spot with bright, indirect light works much better—somewhere the light comes in filtered rather than beating directly onto the glass. In Singapore’s heat especially, it’s worth actually tracking where your terrarium sits throughout the day, not just in the morning.

There’s No Drainage Layer at the Bottom

This is a foundational mistake, and one that’s genuinely hard to fix once the terrarium is already put together.

Without a drainage layer, excess water has nowhere to go. It pools at the base of the soil, slowly suffocates the roots, and creates the kind of stagnant conditions where bacteria quietly takes hold. Starting every build with two to three centimetres of pebbles or lava rocks at the base, followed by a thin layer of horticultural charcoal, gives that excess moisture somewhere to go and keeps the inside of the glass smelling fresh. It’s one of the very first things we cover in our workshops, because without it, even the best plant selection and the most careful watering won’t be enough to save things later on. 

You Used Regular Potting Soil

Standard potting mix is designed to hold onto moisture. This is brilliant for a garden bed, but problematic inside a glass container.

In a terrarium, moisture-retentive soil stays wet far longer than it should, compacts over time, and quietly becomes a breeding ground for fungal issues. For closed, tropical terrariums, a mix of peat, perlite and a small amount of potting mix tends to hit the right balance. For open terrariums with succulents, something sandy and free-draining is what you want. It’s one of those things that feels like a small detail until you realise how much of a difference the right substrate actually makes to everything else.

You Spotted Mould and Decided to Wait

Completely understandable instinct. A small patch of white fuzz shows up and you think you’ll keep an eye on it before doing anything. The problem is that mould in a warm, enclosed space doesn’t stay small for long.

It moves from the soil surface to plant stems quicker than most people expect, and once it gets there it becomes a much harder problem to deal with. Catching it early makes everything easier. Remove the affected material with tweezers, crack the lid open for a day or two to let some air circulate, and ease off watering immediately. If it keeps coming back, a light dusting of cinnamon on the soil surface works well as a natural antifungal.

Nothing Has Been Trimmed in a While

A terrarium is alive, and it grows. Sometimes more quickly than you’d expect. Without the occasional prune, faster-growing plants gradually crowd out the slower ones, blocking light and competing for space both above and below the soil. The balance you carefully put together at the start slowly disappears, and the whole thing starts looking a bit chaotic.

Trimming anything pressing against the glass and removing dead or yellowing leaves as soon as you spot them keeps everything healthier and looking intentional. Once a month is plenty, and honestly it takes about ten minutes.

It’s Been a While Since You Checked On It

This one’s the most human mistake on the list, and probably the most common.

You build it, admire it, maybe show it off to a few people and then life gets busy and the terrarium quietly gets forgotten on a shelf somewhere. Terrariums are truly low-maintenance, but low-maintenance isn’t the same as zero maintenance. Five minutes once a week to check the moisture, glance over the plants, and wipe down the glass is really all it needs. It’s such a small habit, but the difference between catching something minor early and discovering a much bigger problem weeks down the line is significant.

Getting the Basics Right Makes Everything Easier

Most of the mistakes on this list come back to the same thing, which is not having the right foundation in place from the beginning. Once that’s sorted, keeping a terrarium healthy becomes far less complicated than people expect.

That’s really what J2 Terrarium workshops are built around. Whether you’re planning something relaxed with friends or looking for a team activity for a corporate event, we bring the full experience to you, anywhere in Singapore. Our facilitators have over five years of hands-on experience and walk you through every part of the process, covering proper drainage, plant selection, substrate, and what your terrarium actually needs once it’s sitting at home. You leave with something you made yourself, and enough understanding to keep it going long after the workshop is over.

Find a slot for our terrarium workshop.

Quick FAQs

How often should I water a closed terrarium? 

Every two to four weeks is a reasonable starting point, though sometimes even less than that. Light condensation on the glass in the morning is usually a good sign that the water cycle is working fine, meaning you likely don’t need to add more water just yet.

Why are my terrarium plants turning yellow? 

Most of the time it comes down to overwatering, too much direct sunlight, or poor drainage at the base. Checking how often you’ve been watering is usually the best first step.

Is terrarium maintenance difficult? 

Not really, as long as the setup is right from the start. A few minutes of attention each week is genuinely all most terrariums need to stay healthy and looking good.